As a method for controlling a current for a motor equipped with a permanent magnet, a vector control is known. Under the vector control, a current for a motor is separated and controlled into a q-axis current component contributing to torque and a d-axis current component orthogonal to the q-axis current component.
Such a motor control device has been proposed that is capable of performing a precise motor control even when precision in detecting a current lowers in a lower torque, lower speed condition under a conventional vector control (e.g., see PTL 1). FIG. 4 is a block diagram including conventional motor control device 90 appropriate for such a rotation control in a lower torque, lower speed condition.
Conventional motor control device 90 illustrated in FIG. 4 is configured to utilize, as a rotation control unit, vector control unit 92 based on the current vector control in a dq-axis coordinate system, described above, to perform a rotation control on motor 40. In FIG. 4, motor control device 90 is configured to detect drive currents for motor 40 with current detector 12. Detected current detection values Iu, Iv, and Iw are converted by three-phase/two-phase converter 13 configured to perform coordinate conversion into currents in the dq coordinate system, i.e., d-axis current Id and q-axis current Iq. The currents are then supplied to vector control unit 92. Position detector 14 is configured to calculate rotational position Pd indicative of a rotor position of motor 40 based on a signal sent from position detection sensor 49. Rotational speed calculator 15 is configured to calculate and supply, to vector control unit 92, rotational speed Sdet of motor 40 based on rotational position Pd. Vector control unit 92 is further externally supplied with speed instruction Sref instructing a motor speed.
Vector control unit 92 is configured to perform processes described below based on the supplied signals to calculate an instruction voltage for motor drive unit 30. That is, vector control unit 92 first calculates difference dS between speed instruction Sref and rotational speed Sdet. Next, PI computation unit 22 performs proportional integral (PI) computation on difference dS to calculate a torque instruction. From the torque instruction, q-axis current instruction Iq* is further acquired. Difference dIq between q-axis current instruction Iq* and q-axis current Iq is then acquired. PI computation unit 25 performs PI computation on difference dIq to determine and output q-axis voltage instruction Vq* representing a control instruction on a q-axis. On the other hand, d-axis current instruction Id* is specified with a value of “0”, and difference dId between d-axis current instruction Id* and d-axis current Id is acquired. PI computation unit 26 performs PI computation on difference dId to determine and output d-axis voltage instruction Vd* representing a control instruction on a d-axis.
Two-phase/three-phase converter 27 converts q-axis voltage instruction Vq* and d-axis voltage instruction Vd* output from vector control unit 92 into voltage instructions Dru, Drv, and Drw in three phases. Voltage instructions Dru, Drv, and Drw in the three phases are then supplied to pulse width modulation (PWM) circuit 31 of motor drive unit 30. With PWM signals in the UVW phases, which are generated by PWM circuit 31, inverter 32 is controlled to generate and output, to motor 40, drive voltages Vou, Vov, and Vow.
Conventional motor control device 90 illustrated in FIG. 4 further includes, in addition to the ordinary configuration based on the current vector control, described above, offset angle adjustment unit 95 and adder 96, as illustrated in FIG. 4, for a precise motor control. When three-phase/two-phase converter 13 performs coordinate conversion on current detection values Iu, Iv, and Iw, motor control device 90 uses offset angle adjustment unit 95 and adder 96 to add offset angle P of relative to coordinate conversion phases of the currents.
Conventional motor control device 90 added with the above described configuration refrains how torque is to be generated, increases a current for generating identical torque in magnitude as a result, and increases a signal-to-noise (S/N) ratio with respect to a control instruction. In other words, by adding offset angle Pof, less torque is generated than torque instructed by a torque instruction. This leads to an increased torque instruction in order to generate torque at a magnitude similar or identical to a magnitude of torque generated when offset angle Pof is not added. A more current flows accordingly. As a result, precision in detecting the current improves. Conventional motor control device 90 with the above described configuration can perform a precise motor control even when precision in detecting a current lowers in a lower torque, lower speed condition, for example.
Vehicles in which an idle-stop control takes place for improved fuel efficiency are increasing in recent years, for example. In such a vehicle, in addition to an ordinary oil pump that is driven by an engine when the vehicle runs, an electric oil pump that operates during an idle-stop period is also utilized. That is, while the vehicle is stopping and the idle-stop control is taking place, the electric oil pump is driven by a motor to supply hydraulic pressure. A hydraulic pressure force is thus secured while the vehicle is at standstill.
Hydraulic oil used in a hydraulic pump system of a vehicle greatly changes in viscosity due to a temperature. A motor control is required for an electric oil pump so that hydraulic pressure in accordance with a change in oil temperature can be acquired. For example, viscosity of hydraulic oil rises at a lower temperature. A load increases accordingly. Torque greater in magnitude than torque at a normal temperature is thus required. On the other hand, the viscosity of the hydraulic oil lowers at a higher temperature. A load almost disappears accordingly. Even smaller torque can thus secure predetermined hydraulic pressure. In addition, a required flow amount of hydraulic oil during the idle-stop period may be less than a required flow amount of hydraulic oil while the vehicle is running. It is thus advantageous that the motor should rotate at a lower speed during the idle-stop period.
When a temperature is higher during the idle-stop period, a motor of an electric oil pump is required to allow hydraulic oil at lower viscosity, i.e., in an almost no-load condition, to flow at a super low speed. As a result, only a minute current amount is required to properly drive the motor. That is, for a motor of an electric oil pump used in a vehicle and for a motor control device, a motor control capable of performing a precise rotation control even in a lower speed, no-load condition is important. Such a motor control is required that is precise enough even in a lower torque, lower speed condition, as described above.
To satisfy the above described demands on electric oil pumps, such a motor control device has conventionally been proposed that is configured to control a motor so that a rotational speed and torque in accordance with a change in oil temperature are properly acquired (e.g., see PTL 2). The conventional motor control device includes means of generating a control instruction, which is configured to output a q-axis current instruction to a motor, for example. The motor control device controls motor torque and limits a motor rotational speed based on oil temperature information to allow the motor to acquire a rotational speed and torque required by the electric oil pump.
As means of refraining voltage saturation in a vector control, such a flux weakening control is conventionally known that refrains an increase in induced voltage (e.g., see PTL 3). The flux weakening control is implemented with a procedure described below, for example. That is, the control first calculates instruction voltage amplitude value |V*| from instruction voltages Vq*, Vd*, uses a subtractor to subtract instruction voltage amplitude value |V*| from maximum voltage Vlimit, and then calculates voltage error dV*. Next, the control uses a proportional integral controller for flux weakening control to calculate magnetic flux instruction current Idf*, and then adds magnetic flux instruction current Idf* to d-axis instruction current Idx* to perform a flux weakening control.